It is well known in an optical recording system to utilize a single source of electromagnetic energy, such as a laser, to generate a pair of light beams by means of a partially reflective beam splitter. One beam is then modulated and scanned across a light sensitive media to record information thereon. The second beam is scanned across a spatial grating to generate a clock signal which indicates the location of the first beam as it scans across the light sensitive media.
The grating clock signal generated by the second beam allows precise timing control for modulation of the intensity of the first scanning beam and assures precise spatial alignment of the recording signal picture elements from one sweep to the next thus eliminating jitter between succeeding picture elements.
Generation of two beams by use of a partially reflective beam splitter requires additional optics including lenses and mirrors.
In most prior art systems, the recording of information upon a document is accomplished by modulating the intensity of a light beam that scans over the document. In some prior art devices, this modulation is accomplished through the utilization of an acousto-optic modulator. As is known, an acousto-optic modulator passes a beam of light, such as that generated by a laser, through its crystalline structure at the "Bragg" angle and in a generally straight line subject only to refraction caused as the beam passes into the crystalline material and exits therefrom. The exiting beam is commonly referred to as the "zero order" beam. By subjecting the crystalline material to a stimulant, such as acoustic energy created by a transducer mounted on one surface of the crystal, diffraction of the light beam passing therethrough occurs at twice the angle equal to the angle of the incoming beam. This beam is often referred to as the "first order" beam. As a consequence, the intensity of both beams vary with a modulating signal applied to the transducer.
In most prior art facsimile systems, the first order beam is utilized to record information while the zero order beam is not utilized and, in practice, absorbed so as not to interfere with the first order beam. An example of such arrangement is shown in a copending patent application entitled Aperture Stop Assembly For A Laser Beam Optical System by Albert H. Smith, Ser. No. 160,772, filed June 18, 1980, now U.S. Pat. No. 4,290,086, issued Sept. 15, 1981, and assigned to the same assignee as the present invention.